1. Field of the Invention
The present invention relates to a polyurethane binder, electrodes containing the same and a lithium battery employing the electrodes.
2. Description of the Related Art
Lithium batteries have high voltages and high energy densities, and have enhanced stability compared other batteries, such as a Ni—Cd batteries. Due to these advantages, lithium batteries are used as a power source for portable electronic devices. However, since smaller and lightweight portable electric applications become increasingly popular, a battery having a higher operating voltage, longer lifetime, and higher energy density than a conventional lithium battery is desired.
Properties of batteries are dependent on electrodes, electrolytes, and other battery materials included therein. Particularly, properties of electrodes are dependent on electrode active materials, current collectors, and binders that provide a binding force between electrode active materials and current collectors.
A polyvinylidene fluoride (PVDF) based polymer used as a binder material is dissolved in an organic solvent such as N-methyl-2-pyrrolidone when used. The PVDF based polymer has a strong binding force. However, elongation of the PVDF based polymer is very small at about 10%, and thus a large amount of the PVDF based polymer is used to obtain a sufficient binding force thereby limiting the miniaturization. In addition, the PVDF based polymer is dissolved in an organic solvent before use, thereby complicating the manufacturing process. A styrene-butadiene rubber (SBR) used as a binder material has an excellent elastic capacity. However, a binding force of the SBR is very weak such that, after a number of charging/discharging cycles, the structure of an electrode is changed and the capacity and lifetime of a battery including such an electrode is reduced.